1. Field of the Invention
This invention relates to a focus detecting optical device in an optical information recording-reproducing apparatus.
2. Related Background Art
As methods of detecting the focus error of an optical information recording-reproducing apparatus, there are known the knife edge method, the astigmatism method and the critical angle method, and above all, use has been widely made of the knife edge method which is simple in construction and which is not liable to be subjected to disturbance such as diffraction by a guide groove in an optical disk. The principle of this method is shown in FIG. 3 of the accompanying drawings.
In FIG. 3A, a photoelectric detector divided into two (comprising two light receiving elements A and B) is disposed on the focal plane of an objective optical system (of which only the condensing lens 106 is shown) and is positioned so that the dividing line of the detector 108 may coincide with the meridional plane of the objective optical system. A knife edge 107 is disposed between the objective optical system and the detector 108 for movement in a direction perpendicular to the optical axis of the objective optical system. Although not shown, an optical disk is of course disposed at the left of the objective optical system so that a light beam reflected by the recording area forming surface of the optical disk may enter the objective optical system.
Here, consider a case where in the focus detecting optical system of the construction as shown in FIG. 3A, half of the light beam passed through the objective optical system is intercepted by the knife edge 107. First, when the optical disk is in the in-focus position, the imaging point of the reflected light from the optical disk coincides with the dividing line of the detector 108 and the difference (A-B) between the output signals of the light receiving elements A and B becomes 0.
In contrast, when the optical disk is off the in-focus position and is close to the objective optical system, the reflected light therefrom is imaged rearwardly of the detector 108 and the semicircular spot of the light beam shifts toward the light receiving element A side. Therefore, the difference (A-B) between the output signals of the light receiving elements A and B increases in the plus direction. Also, when the optical disk is far from the objective optical system, the reflected light is once imaged short of the detector 108 and then expands and therefore, the semicircular spot shifts toward the light receiving element B side and the difference (A-B) between the output signals of the light receiving elements A and B increases in the minus direction.
This state is shown in the graph of FIG. 3B (wherein the ordinate represents the difference (A-B) between the output signals of the light receiving elements A and B, and the abscissa represents the displacement amount of the optical disk from the in-focus position), and the difference (A-B) between the output signals of the two light receiving elements A and B provides a focus error signal.
In the construction of FIG. 3A, however, half of the light beam is intercepted and this is disadvantageous in respect of sensitivity and therefore, actually, as disclosed, for example, in U.S. Pat. No. 4,712,205, use is made of a focus detecting optical system of such a construction as shown in FIG. 4A of the accompanying drawings.
In the focus detecting optical system of FIG. 4A, instead of half of a light beam being intercepted, a light beam passed through an objective optical system (of which only the condensing lens 206 is shown) is divided into two by a dividing member 207, and the thus divided beams b1 and b2 are received by a detector 208. The detector 208 in FIG. 4A has its light receiving surface divided into four (comprises light receiving elements A, B, C and D), and is disposed on the focal plane of the objective optical system so that the dividing line thereof may coincide with the meridional plane of the objective optical system.
As the dividing member 207, there is known one comprising two wedge-shaped prisms 207a and 207b cemented together as shown in FIG. 4A. The two wedge-shaped prisms 207a and 207b are arranged in a direction orthogonal to the meridional plane of the objective optical system with the meridional plane as the joint surface, and the sides thereof are joined together so that the directions of inclination of the inclined surfaces thereof facing the detector 208 may be opposite to each other.
When in the detecting optical system of the construction as shown in FIG. 4A, an optical disk (not shown) disposed at the left in the plane of the drawing sheet of FIG. 4 is in the in-focus position, the imaging points of the divided two beams b1 and b2 both coincide with the dividing line of the detector 208.
In contrast, when the optical disk deviates from the in-focus position and becomes close to the objective optical system side, the reflected light from the optical disk is imaged rearwardly of the detector 208. Therefore, the semicircular spot of the divided beam b1 shifts toward the light receiving element A side and the semicircular spot of the divided beam b2 shifts toward the light receiving element C side. Also, when the optical disk is far from the objective lens, the reflected light therefrom is once imaged short of the light receiving surface of the detector and then expands and therefore, the semicircular spot of the divided beam b1 shifts toward the light receiving element B side and the semicircular spot of the divided beam b2 shifts toward the light receiving element D side.
Thus, if (A+C)-(B+D) is found from the outputs of the light receiving elements, this value becomes 0 when the optical disk is in the in-focus position, and increases toward the plus side when the optical disk becomes close to the objective optical system side, and increases toward the minus side when the optical disk becomes far from the objective optical system. This state is shown in the graph of FIG. 4B (wherein the ordinate represents the output signal (A+C)-(B+D) and the abscissa represents the displacement amount of the optical disk from the in-focus position), and (A+C)-(B+D) provides a focus error signal.
The adoption of the construction of FIG. 4A leads to the advantage that sensitivity doubles as compared with the case of FIG. 3A and detection is not liable to be affected by the lateral deviation (the deviation in the focal plane) of the detector 208. That is, in the construction of FIG. 3A, when the detector 108 deviates in the direction of division, it directly becomes a factor of a detection error, while in the construction of FIG. 4A, even if the detector 208 deviates in the directions of division of the light receiving elements A and B and the light receiving elements D and C, the error amount (A+B) and the error amount (B+D) of the focus error signal are offset by each other. For example, when the detector deviates toward the light receiving elements B, C side, the semicircular beam correspondingly shifts toward the light receiving elements A, D side and the outputs of the light receiving elements A and D become great, but this affects is no way as the value of (A+C)-(B+D). Accordingly, any detection error is not liable to occur. Also, the four-division detector used in FIG. 4A is readily commercially available and is suitable for practical use.
However, the prior-art focus detecting optical system as described above has suffered from the following problems. Description will hereinafter be made with reference to FIG. 2B of the accompanying drawings showing the manner in which a light beam is divided by a prior-art dividing member.
In FIG. 2B, the entrance surfaces of wedge-shaped prisms 207a and 207b are perpendicular to the optical axis and the exit surfaces thereof are inclined in opposite directions with respect to the optical axis and therefore, a light beam rectilinearly travels in the prisms 207a and 207b, and a beam transmitted through the prism 207a and a beam transmitted through the prism 207b are bent in opposite directions in the exit surfaces and separated from each other. That is, the divided beams b1 and b2 travel obliquely with respect to the optical axis of the objective optical system, and the divided beams form therebetween a certain angle .alpha. conforming to the wedge angle of the prisms 207a and 207b. Thus, the separation spacing S' between the two light spots on the light receiving surface of the detector is proportional to the spacing d between the dividing member 207 and the detector. Accordingly, the dividing member 207 must be placed at such a location that the separation spacing S' is of a value corresponding to the size of the detector, and the location thereof cannot be determined arbitrarily.
Generally, it is often the case that the focal length of the condensing lens of an objective optical system is designed to a great length in order to earn detection sensitivity (to make [the beam shift amount]/[the displacement amount of the optical disk from the in-focus position]) great, and the outer dimension of a four-division detector is often as small as the order of several hundred microns. Thus, the angle .alpha. formed between the divided beams is set to a very small value and the wedge angle of the prisms constituting the dividing member becomes extremely small, and the working tolerance becomes very severe.
If the wedge angle of the prisms is made great (the angle .alpha. formed between the divided beams also becomes great) and the spacing d between the detector and the dividing member is made small, the working tolerance of the prisms will become loose, but the tolerance of the spacing d will now become severe. Also, the divided beams will obliquely enter the detector and detection accuracy will be reduced. Further, in such case, the diameter of the beam transmitted through the dividing member will become very small (because the dividing member is disposed near the focal plane of the objective optical system) and thus, the roughness or the like of the level difference portion (the joint portion of the prisms) will adversely affect detection accuracy.